text: // SPDX-License-Identifier: GPL-2.0-only
  /*
   * Delay loops based on the OpenRISC implementation.
   *
   * Copyright (C) 2012 ARM Limited
   *
   * Author: Will Deacon <will.deacon@arm.com>
   */
  
  #include <linux/clocksource.h>
  #include <linux/delay.h>
  #include <linux/init.h>
  #include <linux/kernel.h>
  #include <linux/module.h>
  #include <linux/timex.h>
  
  /*
   * Default to the loop-based delay implementation.
   */
  struct arm_delay_ops arm_delay_ops __ro_after_init = {
  	.delay		= __loop_delay,
  	.const_udelay	= __loop_const_udelay,
  	.udelay		= __loop_udelay,
  };
  
  static const struct delay_timer *delay_timer;
  static bool delay_calibrated;
  static u64 delay_res;
  
  int read_current_timer(unsigned long *timer_val)
  {
  	if (!delay_timer)
  		return -ENXIO;
  
  	*timer_val = delay_timer->read_current_timer();
  	return 0;
  }
  EXPORT_SYMBOL_GPL(read_current_timer);
  
  static inline u64 cyc_to_ns(u64 cyc, u32 mult, u32 shift)
  {
  	return (cyc * mult) >> shift;
  }
  
  static void __timer_delay(unsigned long cycles)
  {
  	cycles_t start = get_cycles();
  
  	while ((get_cycles() - start) < cycles)
  		cpu_relax();
  }
  
  static void __timer_const_udelay(unsigned long xloops)
  {
  	unsigned long long loops = xloops;
  	loops *= arm_delay_ops.ticks_per_jiffy;
  	__timer_delay(loops >> UDELAY_SHIFT);
  }
  
  static void __timer_udelay(unsigned long usecs)
  {
  	__timer_const_udelay(usecs * UDELAY_MULT);
  }
  
  void __init register_current_timer_delay(const struct delay_timer *timer)
  {
  	u32 new_mult, new_shift;
  	u64 res;
  
  	clocks_calc_mult_shift(&new_mult, &new_shift, timer->freq,
  			       NSEC_PER_SEC, 3600);
  	res = cyc_to_ns(1ULL, new_mult, new_shift);
  
  	if (res > 1000) {
  		pr_err("Ignoring delay timer %ps, which has insufficient resolution of %lluns\n",
  			timer, res);
  		return;
  	}
  
  	if (!delay_calibrated && (!delay_res || (res < delay_res))) {
  		pr_info("Switching to timer-based delay loop, resolution %lluns\n", res);
  		delay_timer			= timer;
  		lpj_fine			= timer->freq / HZ;
  		delay_res			= res;
  
  		/* cpufreq may scale loops_per_jiffy, so keep a private copy */
  		arm_delay_ops.ticks_per_jiffy	= lpj_fine;
  		arm_delay_ops.delay		= __timer_delay;
  		arm_delay_ops.const_udelay	= __timer_const_udelay;
  		arm_delay_ops.udelay		= __timer_udelay;
  	} else {
  		pr_info("Ignoring duplicate/late registration of read_current_timer delay\n");
  	}
  }
  
  unsigned long calibrate_delay_is_known(void)
  {
  	delay_calibrated = true;
  	return lpj_fine;
  }
  
  void calibration_delay_done(void)
  {
  	delay_calibrated = true;
  }
  